Physical Chemistry Chemical Physics

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1 Physical Chemistry Chemical Physics Volume 13 Number June 2011 Pages ISSN COVER ARTICLE Head et al. A computational study on the structures of methylamine carbon dioxide water clusters HOT ARTICLE Donahue et al. Adventures in ozoneland: down the rabbit-hole (2011)13:23;1-X

2 PCCP Dynamic Article Links Cite this: Phys. Chem. Chem. Phys., 2011, 13, PAPER A computational study on the structures of methylamine carbon dioxide water clusters: evidence for the barrier free formation of the methylcarbamic acid zwitterion (CH 3 NH 2 + COO )in interstellar water ices Hakan Kayi, a Ralf I. Kaiser ab and John D. Head* a Received 28th September 2010, Accepted 18th January 2011 DOI: /c0cp01962c We investigated theoretically the interaction between methylamine (CH 3 NH 2 ) and carbon dioxide (CO 2 ) in the presence of water (H 2 O) molecules thus simulating the geometries of various methylamine carbon dioxide complexes (CH 3 NH 2 /CO 2 ) relevant to the chemical processing of icy grains in the interstellar medium (ISM). Two approaches were followed. In the amorphous water phase approach, structures of methylamine carbon dioxide water [CH 3 NH 2 /CO 2 /(H 2 O) n ] clusters (n = 0 20) were studied using density functional theory (DFT). In the crystalline water approach, we simulated methylamine and carbon dioxide interactions on a fragment of the crystalline water ice surface in the presence of additional water molecules in the CH 3 NH 2 /CO 2 environment using DFT and effective fragment potentials (EFP). Both the geometry optimization and vibrational frequency analysis results obtained from these two approaches suggested that the surrounding water molecules which form hydrogen bonds with the CH 3 NH 2 /CO 2 complex draw the carbon dioxide closer to the methylamine. This enables, when two or more water molecules are present, an electron transfer from methylamine to carbon dioxide to form the methylcarbamic acid zwitterion, CH 3 NH 2 + CO 2, in which the carbon dioxide is bent. Our calculations show that the zwitterion is formed without involving any electronic excitation on the ground state surface; this structure is only stable in the presence of water, i.e. in a methyl amine carbon dioxide water ice. Notably, in the vibrational frequency calculations on the methylcarbamic acid zwitterion and two water molecules we find the carbon dioxide asymmetric stretch is drastically red shifted by 435 cm 1 to 1989 cm 1 and the carbon dioxide symmetric stretch becomes strongly infrared active. We discuss how the methylcarbamic acid zwitterion CH 3 NH 2 + CO 2 might be experimentally and astronomically identified by its asymmetric CO 2 stretching mode using infrared spectroscopy. 1. Introduction Scientists have been enamored by the Interstellar Medium (ISM) for more than half a century, particularly after the detection of astrobiologically important molecules such as the sugar glycolaldehyde (HCOCH 2 OH) toward the hot molecular core Sagittarius B2 (Sgr-B2). 1 In the interstellar medium, dense molecular clouds consist predominantly of gaseous molecules (99%) with molecular hydrogen being dominant and ice-coated grain particles of up to a few 100 nm in diameter at a level of typically 1%. 2 5 Astrochemists have a special interest in the origin and formation of amino acids because their presence in the ISM may provide clues to the delivery of prebiotic molecules a Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI 98622, USA. johnh@hawaii.edu b NASA Astrobiology Institute, University of Hawaii at Manoa, Honolulu, HI 96822, USA to the early Earth, the origins of life on Earth, 6 8 and the possibility of Earth-like life elsewhere in the universe. 9 Hitherto amino acids were detected in meteorites such as Murchison, Murray, Orgueil and Ivuna; up to 80 of them were found in the Murchison meteorite alone. 14,15 In extraterrestrial environments, the simplest amino acid glycine (NH 2 CH 2 COOH) was identified in meteorites like Murchison, Orgueil and Ivuna 12,14 andalsoincometarysamplesreturnedwithintheframeworkof the Stardust mission to comet 81P/Wild 2. 16,17 However, the detection of glycine in the interstellar medium has not been confirmed yet. Although Kuan et al. published the first time detection of interstellar glycine, 18 their interpretation was disputed Due to the importance of glycine (NH 2 CH 2 COOH) as the simplest amino acids and as a building block of polypeptides, several different formation mechanisms have been proposed. Proposals for reactions in the gas-phase include gamma- and This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13,

3 X-ray irradiation of carbon-source (carbon monoxide; CO or methane; CH 4 ) and nitrogen-source (nitrogen; N 2 or ammonia; NH 3 )gasmixtures 22 and ion molecule reactions of protonated and ionized hydroxylamine (NH n OH + ; n =2,3) with formic acid (CH 3 COOH). 23 Several other reactions in the solid-state such as proton irradiation of propane (C 3 H 8 ), ammonia (NH 3 ), water (H 2 O), 24 UV irradiation of water (H 2 O),methanol(CH 3 OH), ammonia (NH 3 ), carbon monoxide (CO), carbon dioxide (CO 2 ), 25 UV photolysis of water (H 2 O), methanol (CH 3 OH), ammonia (NH 3 ), hydrogen cyanide (HCN), 26 electron irradiation of methylamine (CH 3 NH 2 ) and carbon dioxide (CO 2 ) ice mixtures, 27 and UV irradiated ice analogs consisting of water (H 2 O), methylamine (CH 3 NH 2 ) and carbon dioxide (CO 2 ) were also proposed. Holtom et al. investigated the formation of glycine (NH 2 CH 2 COOH) and its isomer methylcarbamic acid (CH 3 NHCOOH) via secondary electrons generated in the track of galactic cosmic ray particles in extraterrestrial ices. 27 In their experiments, interstellar ice analogs consisting of a methylamine (CH 3 NH 2 ) and carbon dioxide (CO 2 ) mixture at 10 K were irradiated by 5 kev electrons. Their results indicated the formation of glycine and its methylcarbamic acid isomer at a ratio of about 10 : 1; their findings exposed for the first time detailed reaction mechanisms for glycine and its isomer methylcarbamic acid formation at low temperatures characteristic of interstellar grains in cold molecular clouds such as TMC-1. Further, Lee et al. investigated the effect of UV irradiation on interstellar ice-analog films composed of water (H 2 O), methylamine (CH 3 NH 2 ) and carbon dioxide (CO 2 ). 28 Water dominant samples were exposed to UV light at 56 K. Their results from mass spectroscopic analysis indicated the photochemical synthesis of glycine (NH 2 CH 2 COOH) and its isomers methylcarbamic acid (CH 3 NHCOOH) and methylcarbamate ester (NH 2 COOCH 3 ). Recently, Bossa et al. investigated the thermal reactivity between methylamine (CH 3 NH 2 ) and carbon dioxide (CO 2 ) at low temperatures. 29 They detected methylammonium methylcarbamate [CH 3 NH + 3 ][CH 3 NHCO 2 ] using FTIR spectroscopy. At 248 K from a mass spectrum obtained using a residual gas analyzer they also detected methylcarbamic acid where the most intense peaks in the spectrum were obtained for COO + and CH 3 NH + 2 indicating that gas phase methylcarbamic acid is not stable and decomposes into CH 3 NH 2 and CO 2.Their detailed data analysis suggested that the formation of the glycine salt, methylammonium glycinate [CH 3 NH + 3 ][NH 2 CH 2 CO 2 ], is induced by UV irradiation of the methylammonium methylcarbamate [CH 3 NH + 3 ][CH 3 NHCO 2 ]. In another study, Bossa et al. investigated a water-dominated ice 30 consisting of H 2 O:CO 2 :CH 3 NH 2 in the ratio 10 : 3 : 0.5 and again concluded from FTIR spectroscopy that methylammonium methylcarbamate [CH 3 NH + 3 ][CH 3 NHCO 2 ] is formed which could then be processed by UV radiation to produce the glycine salt. Recently, Goldman et al. 31 suggested that impact-induced shock compression of cometary ices followed by expansion to ambient conditions can produce glycine-containing complexes. Besides the laboratory simulation experiments, the formation of amino acids from simple precursor molecules has also been tackled theoretically. Although numerous computational studies have been conducted on glycine (NH 2 CH 2 COOH) including its neutral and zwitterionic forms in aqueous environments, theoretical studies on methylamine (CH 3 NH 2 ) and carbon dioxide (CO 2 ) interactions are relatively scarce. Andres et al. investigated the addition reaction of carbon dioxide to methylamine at the ab initio level of theory using different basis sets, and examined the relation between transition state structures and reactivity. 42 In related RHF/3-21G calculations, Jamroz et al. investigated the proton transfer from dimethylamine [(CH 3 ) 2 NH] to carbon dioxide (CO 2 ). 43 They found the lowest barrier to proton transfer from the amino group (NHQ) to the carbon dioxide (CO 2 ) was obtained when the transition state calculation was started from a 1 : 2 complex containing one carbon dioxide and two dimethylamine [(CH 3 ) 2 NH] molecules. The lowest energy equilibrium structure for the 1 : 2 complex is cyclic consisting of the carbon atom from carbon dioxide being drawn towards the nitrogen atom on one of the dimethylamine molecules while the second dimethylamine forms a hydrogen bond bridge between an oxygen atom of carbon dioxide and an amine proton on the first dimethylamine. 43 Holtom et al. reported the optimized geometries for the van der Waals complex of methylamine and carbon dioxide (CH 3 NH 2 /CO 2 ) and for different conformers of glycine (NH 2 CH 2 COOH). 27 Recently, Bossa et al. performed calculations to aid in interpreting their experimentally observed barrier of kj mol 1 for the proton transfer from methylamine and carbon dioxide to produce methylcarbamic acid (CH 3 NHCOOH). 29 Just as in the study by Jamroz et al., 43 they also found their calculated barriers to proton transfer were reduced for the (CH 3 NH 2 ) n /CO 2 complexes when n was increased from one to three. The extra methylamine (CH 3 NH 2 ) provides a cooperative binding effect by forming hydrogen bond bridges between the interacting methylamine and carbon dioxide, where the bridging methylamine could also act as a shuttle for the methylamine proton to carbon dioxide. In both studies, the presence of the additional bridging methylamine (CH 3 NH 2 ) or dimethylamine [(CH 3 ) 2 NH] molecules not only decreased the proton transfer activation energies, but they also caused significant bending in the carbon dioxide bond angle and a major decrease in the nitrogen carbon (N C) distance separating the amine group on the methylamine interacting with the carbon dioxide. 29,43 The above calculations suggest that any molecule which can form a hydrogen bond bridge between carbon dioxide (CO 2 ) and methylamine (CH 3 NH 2 ) might draw the two molecules closer together and reduce the barrier for proton transfer between the two molecules. Besides the methylamine carbon dioxide (CH 3 NH 2 /CO 2 ) system, there have been a number of calculations for carbon dioxide in different sized water only clusters and these calculations generally produce optimized structures where the carbon dioxide stays linear except when a proton is transferred to the carbon dioxide. Previously, Khan calculated the bond angle of carbon dioxide in water cluster cages containing 24 and 28 water molecules to be 1791 and 1781, respectively, using HF/6-31G(d) calculations. 44 Jena and Mishra calculated the bending of carbon dioxide in a range between 1.51 and 2.91 for carbon dioxide water clusters [CO 2 (H 2 O) n ; n = 1 6] at different levels of theory for the ground state structures. 45 Peterson et al. concluded that the carbon dioxide could be bent by about 3 41, but it is very unlikely that it is bent by more than 51 according to their Phys. Chem. Chem. Phys., 2011, 13, This journal is c the Owner Societies 2011

4 experimental microwave spectroscopic findings for the carbon dioxide with a two water [CO 2 (H 2 O) 2 ] system. 46 Due to the importance of the methylamine carbon dioxide (CH 3 NH 2 /CO 2 ) system as a potential precursor to glycine and its isomer, we have performed quantum chemistry calculations to simulate the possible interactions taking place between a pair of individual methylamine and carbon dioxide molecules in the presence of a water ice matrix. The waters in these calculations are intended to simulate the dominant water component of interstellar ices in cold molecular clouds. The calculations examine how the methylamine carbon dioxide structural properties and vibrational frequencies evolve in the presence of an increasing number of water molecules. We utilized two different procedures to incorporate water in the calculations. First, we performed geometry optimizations on several different clusters of the type CH 3 NH 2 /CO 2 /(H 2 O) n where n ranged from 0 to 20 and the water has an amorphous structure. Thereafter, we performed calculations on CH 3 NH 2 /CO 2 /(H 2 O) n clusters with n ranging between 52 and 82 of the water molecules arranged as a fragment of the low temperature crystalline ice XI phase structure which is formed below 72 K. The two approaches are highly complementary. The paper is organized as follows. The next section describes the computational methods used along with details on how the different initial CH 3 NH 2 /CO 2 /(H 2 O) n cluster structures were generated. This is followed by the Results and Discussion section where the optimized geometries and computed vibrational frequencies obtained for the amorphous water and crystalline water clusters are presented. Concluding remarks are given in the final section of the paper. 2. Computational method Geometry optimizations were performed first on specific CH 3 NH 2 /CO 2 /(H 2 O) n (n = 0 20) to simulate amorphous water clusters. Typically 20 initial structures for most of the smaller clusters were generated by randomly arranging water molecules around an initially guessed CH 3 NH 2 /CO 2 structure whereas about 10 initial larger cluster geometries were generated by randomly adding extra water to a previously optimized smaller CH 3 NH 2 /CO 2 /(H 2 O) m structure with m o n. While this geometry optimization procedure only samples a small fraction of the possible stable CH 3 NH 2 /CO 2 /(H 2 O) n conformations and should not be expected to find the most stable structure, we found for each n that the three distinct optimized structures with lowest energies had very similar CH 3 NH 2 /CO 2 structural features with the neighboring water molecules. Secondly, to simulate methylamine carbon dioxide interactions in the presence of crystalline water ice, we performed geometry optimizations where the water initial geometry was taken as a fragment of the low temperature (less than 72 K) water ice XI phase which has an orthorhombic crystal structure. The ice XI unit cell contains 8 water molecules and has dimensions a = , b = , c = A. 47 We created a three layer ice slab containing 52 waters using the Avogadro advanced molecular editor 48 and the ice XI unit cell parameters. We then optimized several initial geometries generated with CH 3 NH 2 /CO 2 absorbed at different positions above the slab. From the optimized CH 3 NH 2 /CO 2 /(H 2 O) 52 structure with lowest energy we generated several more initial geometries with 14 and 30 additional waters randomly arranged above the CH 3 NH 2 /CO 2 /(H 2 O) 52 surface and then optimized these structures. All of the geometry optimization and vibrational frequency calculations were performed using the GAMESS program suite. 49 Since performing these calculations using highly correlated methods and/or large basis sets is not practical for our large structures, we used B3LYP density functional theory calculations (DFT) with the 6-31G(d) basis set. 53 To assess the reliability of 6-31G(d) basis for our system, we additionally performed calculations on related structures using the G(d,p) basis set 54,55 on the CH 3 NH 2 /CO 2 /(H 2 O) n clusters containing up to 8 water molecules. Generally we found the 6-31G(d) and G(d,p) basis sets to produce qualitatively similar geometries. Since the main goal of the present study is investigate CH 3 NH 2 /CO 2 structural and vibrational frequency changes with an increasing number of water in the clusters, we considered the 6-31G(d) basis set as a good compromise for providing computational speed with chemical accuracy. Consequently, all of the electronic structure calculations on the larger CH 3 NH 2 /CO 2 /(H 2 O) n cluster with n > 8 were performed at the B3LYP/6-31G(d) level. The large CH 3 NH 2 /CO 2 /(H 2 O) n clusters, which simulate the water structure in a crystalline phase with n Z 52, contained too many atoms to facilitate a large number of different structures to be fully explored with calculations at the B3LYP/6-31G(d) level. To reduce the computational effort we replaced some of the water molecules at the cluster edges, and which were not expected to be directly interacting with or hydrogen bonding to the CH 3 NH 2 /CO 2 complex, with effective fragment potentials (EFP) In the EFP method the chemically inert part (there is no covalent bond breaking process) of the system is replaced by EFP while a regular DFT calculation with contributions from the EFP is performed for the chemically active part such as CH 3 NH 2 /CO 2 with at least the neighboring water molecules. In our calculations we included the surface (upper) layer of the 3-layer ice slab, the CH 3 NH 2 /CO 2 and any other surface water molecules with DFT, and replaced the lower two water layers with the water EFP. Thus in this EFP approach we take the lower two water layers to be inert where the water molecules only interact with each other and the active part through non-bonded interactions. These non-bonding interactions in van der Waals or hydrogen bonded complexes are implemented in the EFP method as: (a) coulombic interactions between fragments, and fragments with quantum mechanical molecules, (b) polarization or induction interaction between fragments, and fragments with quantum mechanical molecules, (c) exchange repulsion and charge transfer terms. 56,60 62 The electrostatic potential of the fragment is included as an additional oneelectron term in the quantum mechanical Hamiltonian used in the active part of the system. 57 Effective fragments have frozen internal geometries. Therefore, they only rotate and translate as they interact with the part of the system treated by the DFT calculation and with each other, but their internal structures do not change during the optimizations. In the CH 3 NH 2 /CO 2 structure determination calculations the lower two layers with the water EFP were constrained and the positions of This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13,

5 fragments were kept fixed while the upper layer was fully optimized. In our ice calculations, we again performed geometry optimizations and vibrational frequency calculations at the B3LYP/6-31G(d) level. For the EFP part of ice, we chose original EFP1/DFT (specifically, B3LYP) potential 59 which is a fitted repulsive potential for water fragments available in GAMESS. 49 After the geometry optimizations were completed, we performed vibrational frequency calculations on the optimized geometries to verify they were proper local minimum and to obtain estimates for the CH 3 NH 2 /CO 2 complex IR vibrational spectra. 3. Results and discussion 3.1 Structural analysis Methylamine and carbon dioxide complex in amorphous water clusters. In this first subsection we describe the optimized geometries we obtained for the clusters where methylamine interacts with carbon dioxide in the presence of water molecules with an amorphous structure. In principle, the methylamine and carbon dioxide interactions might be expected to be the source for producing the neutral or zwitterionic forms of either the simple amino acid glycine or its isomer methylcarbamic acid in extraterrestrial ices. We performed geometry optimization calculations at the B3LYP/6-31G(d) level using different starting geometries on CH 3 NH 2 /CO 2 /(H 2 O) n clusters with n = 0, 1 4, 6, 8, 10, 12, 14, 16, 18 and 20. Fig. 1 shows a schematic representation of the atom numbering used throughout the paper for the CH 3 NH 2 /CO 2 complex and the neighboring water molecules which hydrogen bond with the complex. The lowest energy fully optimized CH 3 NH 2 /CO 2 /(H 2 O) n structures obtained for each n are presented in Fig. 2. We found that in all of the optimized CH 3 NH 2 /CO 2 /(H 2 O) n structures the carbon dioxide favored being closest to the amino end of the methylamine. During the geometry optimizations, we found two different stable arrangements where the carbon Fig. 1 Atom labeling scheme used for the methylamine carbon dioxide complex and neighboring atoms from the water environment. The numbering scheme is: O(2) C(1) O(3) for carbon dioxide, H(6) N(4) H(7) for the amine and C(5) with H(8), H(9) and H(10) for the methyl groups in methylamine; and O(11), O(12) and H(13) to H(18) are atoms on the surrounding water molecules which hydrogen bond to the complex. dioxide was either parallel or perpendicular to the C(5) N(4) bond direction of methylamine. The CH 3 NH 2 /CO 2 /(H 2 O) n structures with n = 2, 3 with carbon dioxide perpendicular to the C(5) N(4) bond were found to be slightly more stable [1.3 (Fig. 2c 0 ) and 4.6 (Fig. 2d 0 ) kj mol 1, respectively] than the parallel carbon dioxide conformers. In all of the other structures, the parallel carbon dioxide conformers were always found to be lower in energy. When we compared the average of C(1) O(2) and C(1) O(3) bond lengths for the different perpendicular and parallel type of conformers in CH 3 NH 2 /CO 2 /(H 2 O) n clusters, we obtained similar average C O distances for the two conformers with the same n. It should be noted that with our geometry optimization strategy we did not expect to find the global minimum for these clusters, but we felt the structural features of these local minima should provide useful insights into how the geometry of the CH 3 NH 2 /CO 2 complex evolves as the number of surrounding water molecules increases. In some of our calculations, we also found some even lower energy CH 3 NH 2 /CO 2 /(H 2 O) n structures, but these structures did not have direct interactions between methylamine and carbon dioxide molecules. Consequently, we do not discuss these structures further here as they do not provide any useful insights into the CH 3 NH 2 /CO 2 interaction. Without any water molecules present (Fig. 2a), we calculated the N(4) C(1) distance between the methylamine N(4) atom and the carbon dioxide C(1) atom to be A and this value is consistent with a van der Waals complex being formed. Tabulated C and N van der Waals radii are 1.55 A and 1.70 A, respectively, 63 and the C(1) atom of carbon dioxide presumably favors interacting with the electron lone pair on the N(4) atom in methylamine. Our geometry is consistent with the previous N C literature values which were calculated to range from and A at different levels of theory for the methylamine carbon dioxide interaction. 27,29,42 In addition, we found the carbon dioxide to be slightly bent with an angle of 1751 also in agreement with the previously reported values between and ,42 Fig. 2 shows that appreciable changes in the CH 3 NH 2 /CO 2 geometry take place as the water molecules are added to the cluster and Table 1 summarizes the numerical geometrical parameters obtained for CH 3 NH 2 /CO 2 in each water cluster. For comparison we also included the B3LYP/6-31G(d) optimized methylcarbamic acid geometrical parameters. Not too surprisingly, we obtained the lowest energy optimized structure with the water molecules preferring to be around the polar end of the complex. It was only after optimizing the CH 3 NH 2 /CO 2 /(H 2 O) 14 cluster that any water molecules were located in vicinity of the methyl group. Table 1 shows that the N(4) C(1) distance between the amino group and carbon dioxide is a primary measure, and the carbon dioxide bond angle, O(2) C(1) O(3), a secondary measure, for how the interactions between methylamine and carbon dioxide change with an increasing number of surrounding water molecules. CH 3 NH 2 /CO 2 /H 2 O. When one water molecule was added to the CH 3 NH 2 /CO 2 complex and optimized, the N(4) C(1) distance was shortened to A and the carbon dioxide bond angle became slightly more bent by 81. Fig. 2b shows that the water acts as a hydrogen bond bridge between carbon Phys. Chem. Chem. Phys., 2011, 13, This journal is c the Owner Societies 2011

6 Fig. 2 Optimized CH 3 NH 2 /CO 2 /(H 2 O) n cluster geometries obtained with amorphous water. The number of water molecules n in the clusters are zero (a), one (b), two (c and c 0 ), three (d and d 0 ), four (e), six (f), eight (g), ten (h), twelve (i), fourteen (j), sixteen (k), eighteen (l) and twenty (m). For clarity, the atoms in the water molecules are represented by using smaller spheres. dioxide and methylamine and formed two hydrogen bonds with the complex: the water O(11) atom with the H(6) atom on the methylamine (2.124 A ) and the water H(1) atom with the O(2) atom on carbon dioxide (2.091 A ). The water serves as a proton acceptor from the amine group on methylamine and a proton donor to an oxygen atom on carbon dioxide. CH 3 NH 2 /CO 2 /(H 2 O) 2. Fig. 2c and c 0 show dramatic changes took place in the CH 3 NH 2 /CO 2 optimized structure when two water molecules were added to the complex. The conformer shown in Fig. 2c 0 is more stable than the conformer in Fig. 2c, but they only differ in energy by 1.3 kj mol 1.In both structures the N(4) C(1) distance was reduced by one angstrom to A and the carbon dioxide became significantly bent with a bond angle of Both CH 3 NH 2 /CO 2 /(H 2 O) 2 conformers formed ring-type structures where again each water acted as a hydrogen bond bridge between the methylamine and carbon dioxide. In the structure shown in Fig. 2c both waters hydrogen bond to just the O(2) oxygen atom in carbon dioxide [H(14) O(2) = A and H(13) O(2) = A ], whereas in Fig. 2c 0 the two water molecules separately form a hydrogen bond with the different oxygen atoms in carbon dioxide [H(13) O(2) = H(17) O(3) = A ]. All of these hydrogen bond distances are shorter than those in the CH 3 NH 2 /CO 2 /H 2 O optimized structure suggesting that the hydrogen bonds in CH 3 NH 2 /CO 2 /(H 2 O) 2 are stronger. This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13,

7 Table 1 Geometrical parameters (bond lengths in Å and bond angles in 1) for the methylamine carbon dioxide complex in the amorphous water clusters, the crystalline water clusters CWI, CWII and CWIII, and for reference the gas phase methylcarbamic acid (MCA, CH 3 NHCOOH), the ions CH 3 NH + 2 COOH, CH 3 NHCOO a along with CO 2 and CO 2 Number of water Carbon dioxide (CO 2 ) NH 2 CO 2 CO 2 CH 3 NH 2 CH 3 O(2) C(1) O(3) C(1) O(2) C(1) O(3) N(4) C(1) C(1) C(5) N(4) C(5) CWI CWII CWIII MCA CH 3 NH + 2 COOH CH 3 NHCOO CO CO a The atom labeling scheme is shown in Fig. 1. Clearly the short A N(4) C(1) distance indicates that the CH 3 NH 2 /CO 2 /(H 2 O) 2 cluster no longer forms just a van der Waals complex with a A N(4) C(1) distance. This short N(4) C(1) distance must arise from the greater amount of hydrogen bonding from the two water molecules, which in turn produces a stronger direct methylamine to carbon dioxide interaction. The optimized CH 3 NH 2 /CO 2 /(H 2 O) 2 geometry suggests an electron from the lone pair on the nitrogen atom is donated to the carbon dioxide, thereby enabling a N(4) C(1) bond to be formed in a zwitterionic complex of the form CH 3 NH 2 + CO 2. The A N(4) C(1) distance appears to be relatively long to be as a covalent bond between the two atoms when compared against the A computed for the N C distance in methylcarbamic acid. Nonetheless, the optimized geometries for the gas phase ions CH 3 NH 2 + CO 2 H and CH 3 NHCO 2 show a lengthening of the N(4) C(1) distance to A and A (Table 1), respectively, supporting the idea that a N(4) C(1) bond is formed in the CH 3 NH 2 /CO 2 /(H 2 O) 2 cluster. Furthermore, when we performed a Mulliken (Lowdin) population analysis on the B3LYP Kohn Sham orbitals for the CH 3 NH 2 /CO 2 /(H 2 O) 2 cluster, we found that there is an appreciable electron transfer from the methylamine to carbon dioxide [0.34 e (0.44 e) and 0.32 e (0.42 e) for the geometries given by Fig. 2c and c 0, respectively] versus the negligible partial charge on carbon dioxide [0.02 e (0.04 e)] computed for the CH 3 NH 2 /CO 2 complex shown in Fig. 2a. There is also a rapid rise in the dipole moment from 2.12 Debye for the CH 3 NH 2 /CO 2 complex with no water to 5.16 Debye for the CH 3 NH 2 /CO 2 /(H 2 O) 2 complex in Fig. 2c 0. In addition, we performed Pipek Mezey orbital localization analysis 64 on the CH 3 NH 2 /CO 2 complex in the gas phase and in the presence of two water molecules and investigated the change in the localized orbitals during the interaction of CH 3 NH 2 and CO 2. The lone pair orbital on nitrogen in the gas phase (Fig. 3a) becomes a C N bonding orbital when two waters are added to the system (Fig. 3b). These results suggest that the two waters enable the CH 3 NH 2 and CO 2 to combine to produce the zwitterionic form of methylcarbamic acid, CH 3 NH + 2 CO 2. In addition, we find as expected that one role of the water is to reduce the energy needed to remove an electron from CH 3 NH 2 and add an electron to CO 2. For CH 3 NH 2, we compute an ionization energy of 8.55 ev in the gas phase and 7.54 ev with two waters; whereas 1.35 ev is needed to add an electron to gas phase CO 2 which is reduced to 0.13 ev with two waters present. Thus we compute a 2.23 ev net energy reduction in favor of forming the ions in presence of two water molecules. However, these energy calculations need to be considered with caution because there are large error bars associated with the large conformational changes in the explicit water geometries as the neutral CH 3 NH 2 and CO 2 molecules become charged and because the resulting CO 2 geometry is bent with a bond angle. The 0.05 A increase in the average C O bond length and bond angle in carbon dioxide with two waters are also consistent with CO 2 starting to be formed. To further evaluate whether the role of the bridging water molecules is just to pull the carbon dioxide and methylamine closer together, which then facilitates electron transfer from the methylamine to carbon dioxide, we performed an electronic structure calculation on the optimized CH 3 NH 2 /CO 2 /(H 2 O) 2 geometry but with the two waters removed. A possible alternative viewpoint is that the bridging water molecules cause a polarization of the CH 3 NH 2 /CO 2 complex necessary to facilitate the electron transfer. From our calculations we found that the energy of the CH 3 NH 2 /CO 2 structure from Fig. 2c 0 but without Phys. Chem. Chem. Phys., 2011, 13, This journal is c the Owner Societies 2011

8 Fig. 3 Localized orbitals showing the interaction between CH 3 NH 2 and CO 2 (a) in the gas phase and (b) in the presence of two water molecules. the 2 waters increased by 62 kj mol 1 relative to the fully optimized CH 3 NH 2 /CO 2 structure given in Fig. 2a. But we also found the unoptimized structure (Fig. 2c 0 without waters) to have a 6.72 D dipole moment and the Mulliken (Lowdin) population analysis net carbon dioxide charges [0.34 e (0.45 e)] much greater than the 2.13 D dipole and carbon dioxide charge [0.02 e (0.04 e)] for the optimized CH 3 NH 2 /CO 2 van der Waals complex. These calculations support the idea that the bridging water molecules are responsible for bringing the carbon dioxide and methylamine closer together, and this then enables a substantial electron transfer from methylamine to carbon dioxide. A plot of the extent of electron transfer from methylamine to carbon dioxide as suggested by a population analysis against the number of water molecules is shown in Fig. 4a. Likewise the dipole moment variation computed for the CH 3 NH 2 /CO 2 structure taken from the optimized CH 3 NH 2 /CO 2 /(H 2 O) n cluster shown in Fig. 4b is also consistent with electron transfer from methylamine to carbon dioxide when n = 2. We conclude that the two bridging water molecules are able to promote electron transfer thermally without requiring some electronic excitation process to stimulate the electron transfer. CH 3 NH 2 /CO 2 /(H 2 O) Including more waters, from 3 (Fig. 2d) to 12 (Fig. 2i), continues to more gently modify the CH 3 NH 2 /CO 2 complex structure. In Fig. 5, we graphed the variation in the N(4) C(1) distance between methylamine and carbon dioxide, the CO 2 bond angle and the two C O bond lengths as a function of the number of waters in the CH 3 NH 2 /CO 2 /(H 2 O) n complex. Fig. 5 shows the N(4) C(1) distance decreases gradually from A to A, the Fig. 4 Variation of (a) the carbon dioxide net charges (in e) obtained by the Mulliken and Lowdin populations analyses, and (b) CH 3 NH 2 /CO 2 dipole moment (in D) computed without the water present plotted against the number of water molecules in the CH 3 NH 2 /CO 2 /(H 2 O) n clusters. carbon dioxide angle changes from 1381 to 1311, and the C O bond lengths are slightly lengthened on going from 3 to 12 waters. At the same time, both increases in the dipole moment (7.06 to 8.04 D) and the Mulliken (Lowdin) population analysis of carbon dioxide charge [0.36 e (0.46 e) to 0.45 e (0.56 e)] indicates the amount of electron transfer from the methylamine to carbon dioxide increases slightly after more than two waters are added to the cluster (Fig. 4). The carbon dioxide bond lengths and bond angles for n = 3 to 20 are very similar to the A C O bond length and O C O bond angle obtained by separately optimizing the CO 2 anion at the B3LYP/6-31G(d) level. It is interesting to note the hydrogen bonding water bridges pulling carbon dioxide closer to methylamine in the CH 3 NH 2 /CO 2 /(H 2 O) n complexes with n > 2 in some cases contain more than a single water molecule. For example, for three waters in Fig. 2d and d 0, one of the hydrogen bond bridges is made up from two water molecules, and the two hydrogen bond bridges between carbon dioxide and methylamine for the four water structure in Fig. 2e both consist of two water molecules. The presence of two waters in the hydrogen bonding bridge enables the N(4) C(1) distance This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13,

9 Fig. 5 Graphs of (a) the methylamine carbon distance given as the N(4) C(1) distance (in Å), (b) the carbon dioxide bond angle (in 1), and (c) the two C O bond lengths and the average C O distance in carbon dioxide (in Å) plotted against the number of water molecules in the CH 3 NH 2 /CO 2 /(H 2 O) n cluster. to be shortened further from A (n = 3) and A (n = 4). (n = 2) to A CH 3 NH 2 /CO 2 /(H 2 O) Table 1 and Fig. 5 show the CH 3 NH 2 /CO 2 structure to remain essentially constant in the clusters containing 14 (Fig. 2j) to 20 (Fig. 2m) water molecules. This is partly because the extra water molecules in the 14 to 20 structures are being added around the methyl end of methylamine and away from the region where the carbon dioxide interacts with methylamine. Nevertheless, we obtained the shortest N(4) C(1) distance (1.512 A ) and the smallest carbon dioxide angle (1301) with the cluster containing 20 water molecules. Fig. 5c shows that there can be a fairly wide difference between the two C(1) O(2) and C(1) O(3) distances in carbon dioxide especially in the n = 2 to 10 clusters. We attribute these differences to O(2) and O(3) atoms interacting with different numbers of surrounding water molecules and found the average C O distance to increase smoothly to 1.25 A when there are twenty water molecules in the cluster. In the n = 20 case, the water molecule arrangement around each of the oxygen atoms on the bent CO 2 causes the two C O distances to be essentially the same. Fig. 6 summarizes the hydrogen bond distance variation with number of water molecules in the cluster. From Fig. 6 we again found initially large distance changes as the first 2 to 4 waters are added to the CH 3 NH 2 /CO 2 complex, and then a more gentle variation in the hydrogen-bond lengths for the larger clusters. Our calculations suggest that the bridging water molecules which form hydrogen bond bridges between methylamine and carbon dioxide appear to be facilitating a thermal electron transfer from methylamine to carbon dioxide to presumably end up with a methylcarbamic acid zwitterion of the form CH 3 NH 2 + CO 2. There were a few examples of related systems reported previously: cyanamide water [NH 2 CN/(H 2 O) n ; n = 1 6), 65 dimethylamine carbon dioxide [((CH 3 ) 2 NH) n /CO 2 ; n =1,2], 43 ammonia carbon dioxide [(NH 3 ) n /CO 2 ; n =1,2], 66 and methylamine carbon dioxide [(CH 3 NH 2 ) n /CO 2 ; n = 1 3] systems 29 where one of the molecules acts as a hydrogen bond bridge molecule. One interesting aspect of the CH 3 NH 2 /CO 2 /(H 2 O) n cluster geometries is that the carbon dioxide becomes more bent as more waters are added to the clusters. This contrasts with carbon dioxide on its own in water clusters where the carbon dioxide essentially remains linear. 44,45 We discuss the consequences of the carbon dioxide bending on the vibrational frequencies for CH 3 NH 2 /CO 2 /(H 2 O) n in more detail below. We found the CH 3 NH 2 + CO 2 zwitterion is only stable in the presence of the surrounding water molecules. When we start a geometry optimization using an initial CH 3 NH 2 + CO 2 structure taken from one of the optimized CH 3 NH 2 /CO 2 /(H 2 O) n clusters but with the n water molecules removed we always obtain the van der Waals complex structure shown in Fig. 2a. A similar geometry optimization result was found for glycine, where the optimized glycine zwitterion is only computed when water molecules are hydrogen bonded to the glycine. 37,40,67 Finally, we note that in all of these calculations we never found a structure where a hydrogen atom spontaneously transferred from methylamine to carbon dioxide. Presumably this proton transfer will require, as proposed by others, 40,41 some excitation process, such as by UV or electron irradiation, before this can occur. However, the optimized structures we found from our calculations do suggest that the hydrogen bonded bridging waters could perhaps act as a shuttle for the proton transfer from methylamine to carbon dioxide Phys. Chem. Chem. Phys., 2011, 13, This journal is c the Owner Societies 2011

10 surface layer of the ice slab, while 36 of the water molecules located in the lower two layers of the ice were treated using the EFP1/DFT potentials. 59 The geometries of the three structures were partially optimized by keeping the 36 EFP waters fixed at the experimental ice XI geometry and minimizing the clusters energy by relaxing the positions of all the other atoms which are included in the DFT calculation. We found the resulting optimized structures interesting in that, although the top surface layer of the ice can fully relax, the cluster retains most of its crystalline structure. Our approach is consistent with a previous study which found that when a full geometry optimization of a cluster simulating crystalline water is performed, the resulting geometry is dramatically deformed at the cluster edges due to the formation of extra hydrogen bonds not present in the bulk structure. 73 Fig. 6 Variation of the hydrogen bond distance (in A ) between neighboring water molecules and (a) the oxygen atom on carbon dioxide and (b) the hydrogen atoms on the methylamine amine group Methylamine and carbon dioxide complex on a crystalline water ice surface. In this part, we report the results from the investigation of the CH 3 NH 2 /CO 2 complex adsorbed on a crystalline water ice surface. We also considered the situation where there were enough additional water molecules to completely surround the CH 3 NH 2 /CO 2 complex on the ice surface. Although mostly the amorphous phase of water ice was previously considered in experimental studies and theoretical models of icy grain mantels, Rimola et al. 73 and Allouche et al. 74 have used crystalline ice structures in their cluster calculations after making the assumption that amorphous ice can be regarded as being partially ordered on the molecular scale. As described in the Computational method section, we prepared a small fragment of the ice XI crystal structure composed of 52 water molecules arranged as a slab simulating the top 3-layers of the (100) ice XI surface built using geometry information based on the ice XI crystallographic data. 47 Initial guesses at the preferred CH 3 NH 2 /CO 2 complex adsorption geometry were made by placing the complex with a 3 A N(4) C(1) distance at different sites at approximately 5 A above the surface. A partial geometry optimization calculation was performed on each CH 3 NH 2 /CO 2 /(H 2 O) n structure with n = 52 (CWI), 66 (CWII), and 82 (CWIII) at the B3LYP/ 6-31G(d) level for methylamine, carbon dioxide and the water CWI. Fig. 7 shows the resulting lowest energy optimized structure of CH 3 NH 2 /CO 2 /(H 2 O) 52 (CWI) containing the CH 3 NH 2 /CO 2 complex at its preferred adsorption site above the water ice surface. We found that each of the oxygen atoms in carbon dioxide make a hydrogen bond with a water molecule in the ice surface and the amino group of methylamine also makes a hydrogen bond to an oxygen atom on a different water in the ice surface. We calculated the carbon dioxide O(2) H(14), O(3) H(16) and amino H(7) O(12) hydrogen bonding distances to be A, A and A, respectively. A summary of the CH 3 NH 2 /CO 2 complex geometry contained in CWI is included in Table 1. The large 391 bending of the carbon dioxide bond angle and the average C O distance of A along with the reduced carbon dioxide to methylamine separation to produce a N(4) C(1) distance of A suggests that the geometry of CH 3 NH 2 /CO 2 complex within the CWI structure is very similar in nature to that occurring for the CH 3 NH 2 /CO 2 /(H 2 O) 2 amorphous cluster. An important difference in all the crystalline water clusters is that the carbon dioxide and methylamine can more readily form hydrogen bonds with several different water molecules. This contrasts with the hydrogen bonds in the amorphous clusters which have two or three water molecules which simultaneously hydrogen bond to both the carbon dioxide and methylamine molecules. CWII. To investigate whether the interactions in the CH 3 NH 2 /CO 2 complex adsorbed on the ice surface become modified by the presence of extra surface waters we performed a geometry optimization on the CH 3 NH 2 /CO 2 /(H 2 O) 66 cluster (CWII) with 14 extra water molecules added to the CWI structure shown in Fig. 7. This number of extra waters in CWII was selected so as to enable the sides of the CH 3 NH 2 /CO 2 complex to be completely surrounded. Once again, we performed a geometry optimization using EFP1/DFT potentials for the fixed water molecules in the lower two layers of the ice and a B3LYP/6-31G(d) calculation for the rest of the system. The CWII optimized structure is shown in Fig. 8 with the CH 3 NH 2 /CO 2 complex geometry summarized in Table 1. The 14 additional waters formed a new ice layer with a hydrogen bonding network with the ice surface and also with the CH 3 NH 2 /CO 2. Analogous to the CH 3 NH 2 /CO 2 /(H 2 O) n amorphous clusters, the additional water surrounding the This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13,

11 Fig. 7 Adsorption of methylamine and carbon dioxide on top of the ice XI surface consisting of 16 waters and 36 EFP waters: (a) side-view, and (b) top-view. For clarity, the atoms in the water molecules are represented by using smaller spheres. CH 3 NH 2 /CO 2 complex in CWII appears to make the interaction between the methylamine and carbon dioxide stronger. This is indicated by the much shorter N(4) C(1) distance of A, a longer average A C O distance and the smaller 1331 bond angle relative to CWI. The N(4) C(1) distance and the distorted carbon dioxide geometry in CWII suggest that the CH 3 NH 2 /CO 2 complex resembles the amorphous cluster containing 8 waters given in Table 1. CWIII. Finally, we formed the CWIII structure by adding 16 water molecules to the top layer of the CWII structure (or 30 to the CWI structure) and performed the same type of partial geometry optimization as before. The CWIII optimized geometry is shown in Fig. 9 and now has the CH 3 NH 2 /CO 2 complex completely encapsulated in a hydrogen bonding network formed by the ice surface and the surrounding water molecules. Again, the fully trapped complex has a shorter N(4) C(1) distance (1.523 A ), longer average C O distance (1.248 A ) and smaller carbon dioxide angle (1311) than in the CWII structure suggesting that in CWIII the methylamine and carbon dioxide interact with each other more strongly than in CWII. The geometrical parameters of the CWIII structure given in Table 1, where the CH 3 NH 2 /CO 2 complex is completely encapsulated by water molecules, resemble the complex in the amorphous cluster containing 16 water molecules. Overall, we feel the trends in the CH 3 NH 2 /CO 2 complex geometrical parameters summarized in Table 1 obtained from Fig. 8 Methylamine carbon dioxide in the top layer of the ice XI surface surrounded by 14 additional water molecules around the sides so that the cluster includes 30 waters and 36 EFP waters: (a) side-view, and (b) top-view. For clarity, the atoms in the water molecules are represented by using smaller spheres. the calculations on the CH 3 NH 2 /CO 2 /(H 2 O) n clusters with amorphous water and the crystalline water demonstrate that the extent of the methylamine and carbon dioxide interaction mainly depends on the number of surrounding water neighbors. We found the CH 3 NH 2 /CO 2 complex in the crystalline water clusters CWI, CWII and CWIII to resemble the amorphous clusters containing n = 2, 8 and 16 waters, respectively, indicating that the N(4) C(1) distance is not governed by a specific type of hydrogen bonding bridge formed by one or more water molecules. This also suggests that forming the CH 3 NH 2 /CO 2 complex is not sensitive to being in an amorphous or crystalline water environment. Dipole moment calculations, where the surrounding water molecules are removed, gave 6.63, 7.80, 8.07 Debye, respectively, for the CWI, CWII and CWIII optimized CH 3 NH 2 / CO 2 complexes. Both the dipole moment calculations and the Mulliken and Lowdin population analyses are consistent with a large electron transfer from the methylamine to carbon dioxide taking place to produce the methylcarbamic acid zwitterion CH 3 NH + 2 CO 2 in the crystalline water ice structures Phys. Chem. Chem. Phys., 2011, 13, This journal is c the Owner Societies 2011

12 carbon dioxide fundamentals correlated well with the experimental gas phase vibrations (IR active intense asymmetric stretch 2349 cm 1 and bending 667 cm 1, and the IR inactive symmetric stretch 1333 cm 1 ). 76 Likewise, the computed fundamentals for the isolated methylamine generally agreed well with the previous experimental IR active frequency assignments listed in Table 2 obtained for solid methylamine. 27,75 Perhaps the major discrepancy in the computed frequencies for the isolated methylamine molecule occurred for the NH 2 n 1 symmetric and n 10 asymmetric stretches where they exceed the experimental frequency values by approximately 200 cm 1. But as we discuss below, these computed NH 2 stretching frequencies become much closer in agreement with the experimental values in the CH 3 NH 2 /CO 2 /(H 2 O) n complexes when water molecules form hydrogen bonds with the NH 2 groups. Fig. 9 Methylamine carbon dioxide trapped encapsulated in ice formed from 46 waters and 36 EFP waters: (a) side-view, and (b) top-view. For clarity, the atoms in the water molecules are represented by using smaller spheres. 3.2 Vibrational frequency analysis In this section, we present the results from B3LYP/6-31G(d) vibrational frequency calculations on the geometry optimized amorphous and crystalline CH 3 NH 2 /CO 2 (H 2 O) n clusters described in the last two subsections. Table 2 summarizes the frequencies computed for isolated methylamine and carbon dioxide molecules (in column 3), the CH 3 NH 2 /CO 2 van der Waals complex (column 4) and the two CH 3 NH 2 /CO 2 /(H 2 O) 2 (columns 5 and 6) conformers. Also included in Table 2 are the experimentally observed IR active vibrational frequencies of Holtom et al. 27 and Bossa et al. 75 for methylamine carbon dioxide binary ice mixtures at 10 K. In the following discussion we used the computed vibrational frequencies as directly obtained from the B3LYP/6-31G(d) calculations rather than with a 0.96 scaling factor often used to compensate for the theoretical frequencies tending to be systematically higher than the experimental values. 76 The computed fundamental vibrations in Table 2 from the calculations on the individual methylamine and carbon dioxide molecules essentially simulate the frequencies observed for the isolated molecules in the gas phase. The computed CH 3 NH 2 /CO 2. In the CH 3 NH 2 /CO 2 van der Waals complex (Fig. 2a) there is still a relatively large separation between the carbon dioxide and methylamine, and consequently there are only small shifts (o20 cm 1 ) in the vibrational frequencies from those for the isolated carbon dioxide and methylamine molecules. The slight 51 distortion from linear carbon dioxide in the complex caused the doubly degenerate 640 cm 1 bending mode to split into a bending mode (647 cm 1 ) in the carbon dioxide place and an out-of-plane bending mode (595 cm 1 ) which are both IR active. Since the carbon dioxide is no longer linear, the out-of-plane bending mode more strictly corresponds to a frustrated carbon dioxide rotation which couples with vibrational modes involving the nitrogen atom of methylamine to produce an IR active band with a greater intensity ( cm molecule 1 ) than the in-plane carbon dioxide bending vibration ( cm molecule 1 ). Another important consequence of the carbon dioxide bending is that the originally IR inactive carbon dioxide symmetric stretch at 1371 cm 1 starts to become weakly IR active ( cm molecule 1 ) in the CH 3 NH 2 /CO 2 complex. CH 3 NH 2 /CO 2 /(H 2 O) 2. The fifth and sixth columns in Table 2 list the computed frequencies for the parallel and perpendicular conformers of the CH 3 NH 2 /CO 2 /(H 2 O) 2 complex. This complex can be regarded as a prototype for the larger interactions which take place between methylamine and carbon dioxide in the presence of several water molecules in a low temperature ice. As we have already noted, the two water molecules draw the carbon dioxide approximately an A closer to the nitrogen atom in methylamine. The resulting larger methylamine and carbon dioxide interaction seems to mostly affect the carbon dioxide vibrational modes. The carbon dioxide asymmetric stretch remains strongly IR active but is drastically red shifted by 435 cm 1 to 1989 cm 1. The carbon dioxide symmetric stretch is less strongly red shifted by 75 cm 1 to 1289 cm 1, however, since the O C O angle is now bent to 1401 there is a huge increase in the n s IR band intensity. The carbon dioxide bending mode is blue shifted in the parallel (perpendicular) conformers by 30 cm 1 (76 cm 1 ). The different symmetries of the two CH 3 NH 2 /CO 2 /(H 2 O) 2 structures result in a much larger bending vibration IR intensity in the perpendicular versus the parallel CO 2 conformer. This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13,